Feature

There is growing interest in new devices based on confined states in quantum wells. Optical transitions between subbands are of special interests because they can be tuned over most of the far- and mid-infrared spectrum. Spontaneous emission from transitions between subbands in isolated quantum wells has been observed by Helm et al. by electrically heating the electrons. We describe here the observation of radiative intersubband transitions by optically excited carriers in a quantum-well structure.